Non-Formaldehyde based electroless Cu deposition for Advanced Packaging

Abstract

A non-formaldehyde electroless Cu bath is evaluated in this work. Sodium hypophosphite is used as the reducing agent and electroless Cu deposition is done on Al pads using zincation as activation process. Other than copper sulfate as the copper ion source and sodium hypophosphite as the reducing agent, sodium citrate and a small amount of Ni salt are also used as the complexing agent and catalyst, respectively. The effects of pH, temperature and boric acid concentration as the buffering agent on the deposition rate and quality are evaluated. A good quality of the Cu deposits could be achieved at pH 10, temperature 70°C and boric acid concentration of 1-2 g/L at high deposition rate (200-300 nm/min). Boric acid is also found to facilitate the Cu deposition. SEM images show a uniform and a fully covered Cu layer formed on all the pads. FIB cross section images depict amorphous nature of the deposit. A solder ball bumping of Pb-free SnAgCu solder alloy was evaluated for characterization of the interfacial reaction of the deposited Cu and the solder alloy. Good adhesion and shear results observed for the bumped solder balls on the deposited Cu pads. Interfacial strength and reliability of bumped solder alloy on the pads were evaluated using multiple reflow.